Waveform independent coarse synchronization method

ABSTRACT

A wideband chaotic waveform that is rateless in that it may be modulated at virtually any rate and has a minimum of features introduced into the waveform. Further, the waveform provided may be operated below a signal to noise ratio wall to further enhance the LPD and LPE aspects, thereof. Additionally, the present disclosure may provide a mix of coherent and non-coherent processing techniques applied to signal samples to efficiently achieve coarse synchronization with a waveform that is faster, more efficient and more accurate than using time domain signal correlators alone.

TECHNICAL FIELD

The present disclosure relates generally to low probability of detectionand low probability of exploitation communications. More particularly,in one example, the present disclosure relates to methods of waveformgeneration and waveform synchronization for use with low probability ofdetection and low probability of exploitation communications.Specifically, in another example, the present disclosure relates tomethods of generation for wideband, featureless, and rateless chaoticwaveforms and to methods of waveform coarse synchronization withoutregard to the type of waveform being synchronized.

BACKGROUND

Radio communications, particularly those used in military operationstaking place in contested or hostile environments, are often vital tothe success of such operations. Such communications may be used locally,for example, to coordinate troop positions or movements as well as torelay and/or exfiltrate information out to or from a remote locationsuch as a base of operations or command center. Each of these specifictypes of communications may have different requirements relating topower, frequency, or the like. Similarly, these communications may havedifferent requirements and/or objectives depending on the particulars ofthe mission or operation at hand.

As these battlefield communications are usually vital to the missionand/or to the safety of the mission participants, adversaries in thearea of operations may look to exploit such communications to determineinformation about their opponent. For example, adversaries may use theexistence of communication signals to determine information such as thelocation, movement, and/or type of vehicle or unit deployed. Further,hostile adversaries may use this information for offensive or defensivemeasures, such as for missile guidance or the like. Accordingly, it isdesirable to utilize low probability of detection (LPD) and/or lowprobability of exploitation (LPE) communications to thwart such uses ofthese communications signals. As adversarial sensor capabilitiesimprove, these LPD/LPE technologies become more important; however, theytypically come at the cost of reduced throughput, which may furtherimpact mission effectiveness. It is therefore increasingly important tooperate or generate LPD/LPE signals to evade even the most advanceddetectors.

Current detectors tend to operate under principles of energy detection,waveform feature detection, and/or signal rate detection. While theseare not the limit of the types of detectors that exist, energy detectorsare the most basic and most common detector currently in use. The basicprinciple of operation for energy detectors is to monitor or “listen”for signals across the electromagnetic spectrum (or particular frequencybands thereof). Energy detectors are capable of near continuousoperation if desired and the longer these detectors “listen” the morelikely they are to detect a signal. Thus, if they “listen” long enough,these detectors will theoretically always detect any signal generated ata finite rate. Thus, according to information theory, LPD is impossiblegiven a long enough integration time.

Accordingly, if a signal is generated that can avoid detection by anenergy detector with a sufficiently long integration time, that samesignal will avoid all other types of detection and/or minimize theprobability of detection by other types of detectors based on not beingdiscovered in the first place. In other words, if the energy detectorscan't determine a signal is there, other detectors focused on certainaspects of that signal (e.g. rate, features, etc.) won't know where tolook to find that signal either.

One method, discussed further below, that may be useful for LPD/LPEcommunications involves the use of a cognitive system utilizing areasoning element to allow for dynamic changes to mission parameters andmission communications in real time. One such example with commoninventorship hereto may be found in U.S. patent application Ser. No.16/804,104, filed Feb. 28, 2020, the disclosure of which is incorporatedby reference as if rewritten herein.

Such solutions may utilize a cognitive radio system having a strategyoptimizing component as a reasoner utilizing gain theory to reason overa set of policy constraints over the system. These constraints maycontrol the actions of the radio system utilized in LPD/LPEcommunications and may include determinations relating to aspects suchas frequency bands relative to time, placement, and/or direction of thetransmission of data, range and position of enemy detectors, and/orrange and position of the transmitting unit relative to receiving units.While such systems may enable LPD/LPE communications to occur, the form,or more particularly, the waveform of those communications may still bedetectable in certain scenarios, such as instances where the detector ismore advanced or more agile.

Other LPD/LPE systems tend to rely on spread spectrum techniques tointroduce features into the waveform to reduce detectability. However,these introduced features may still permit some level of detectabilityfor the waveform by some particular detectors. Alternatively, chaoticwaveforms may be employed; however, they tend to be more narrowly bandedand commonly “hop” to fully utilize the width of the spectrum. Suchhopping may likewise introduce features into the waveform, which againmay permit some level of detectability therefor. Similarly, currentchaotic waveforms often operate at a limited number of data rates, whichagain may increase the detectability thereof.

Additional issues exist for LPD/LPE waveforms, particularly relating tothe synchronization of a receiver with a transmitted waveform. Inparticular, as waveforms evolve and become more complex to avoiddetection, their effectiveness as communications signals may suffer dueto synchronization difficulties on the receiving end. Put another way,even if your signal is not detectable by adverse receivers, the signalis of little to no value if it cannot be synchronized and receivedcoherently by your target receiver.

Signal synchronization poses a particularly difficult challenge whenspecific characteristics of the signal such as modulation and coding arenot known. In other instances, the channel between the signal source andsignal receiver may distort the waveform in various ways, includingunknown time and frequency shifts. In both of these cases, it becomesnecessary to efficiently search time and frequency spaces for thewaveform (typically done using a time domain signal correlator) tocorrectly identify the time and frequency parameters of the channel.Only then can the receiver be synchronized with the waveform. Often,this initial degree of synchronization is less accurate than astypically required to fully process the received waveform.

Thus, it is difficult to efficiently achieve coarse synchronization of aLPD/LPE waveform with a given set of time and frequency samples.Further, other effects, such as wideband Doppler, may cause distortionduring reception of the signal samples that may further limit theeffectiveness of a time domain signal correlator.

SUMMARY

The present disclosure addresses these and other issues by providing awideband chaotic waveform that is rateless in that it may be modulatedat virtually any rate, has a continually varying rate, and has a minimumof features introduced into the waveform. Further, the waveform providedmay be operated below a signal to noise ratio wall to further enhancethe LPD and LPE aspects, thereof.

Additionally, the present disclosure addresses these issues through amix of coherent and non-coherent processing techniques applied to signalsamples to efficiently achieve coarse synchronization with a waveformthat is faster, more efficient and more accurate than using time domainsignal correlators alone.

In one aspect, an exemplary embodiment of the present disclosure mayprovide a method of waveform synchronization comprising: receiving asignal via at least one antenna and at least one receiver, the signalhaving a known preamble and wherein the data used in generating thesignal are not known except for the data symbols in the preamble;creating a plurality of coherence domains using time and frequencyconstraints to accept maximum uncompensated Doppler values; establishinga plurality of FFT bins in each of plurality of coherence domains;performing a series of bulk correlations on the received signal bymultiplying the received signals with a reference signal derived fromthe known preamble in the frequency domain and applying an inverse fastFourier transform (FFT) to the result to estimate the correlation valuesat points in time; detecting and isolating the preamble of the receivedsignal; estimating the synchronization values of the received signalwithin the coherence domains based on the known preamble; estimating thesynchronization values of the received signal between coherence domainsbased on the known preamble; and updating the synchronization values ofthe received signal on an ongoing and real time basis using theestimated synchronization values from the detected signal from a postprocessing feedback loop. This exemplary embodiment or another exemplaryembodiment may further provide wherein estimating the synchronizationvalues of the received signal within coherence domains is performedusing coherent processing techniques. This exemplary embodiment oranother exemplary embodiment may further provide wherein estimating thesynchronization values of the received signal between coherence domainsis performed using non-coherent processing techniques. This exemplaryembodiment or another exemplary embodiment may further provide whereincreating the plurality of coherence domains further comprises:calculating a size constraint in the time domain based the highestDoppler frequency of concern determined from a maximum uncompensatedvelocity between a transmitter generating the detected signal and thereceiver. This exemplary embodiment or another exemplary embodiment mayfurther provide wherein the value of the size constraint in the timedomain is approximately one quarter of a period of the highest Dopplerfrequency of concern. This exemplary embodiment or another exemplaryembodiment may further provide wherein creating the plurality ofcoherence domains further comprises: calculating a size constraint inthe frequency domain based on a maximum Doppler frequency offset that isless than the FFT bin size determined by a drift limit of less than180°; and calculating the number of coherence domains required to keep aDoppler skew below 10°. This exemplary embodiment or another exemplaryembodiment may further provide wherein the size constraint in thefrequency domain is approximately one tenth of the FFT bin sizedetermined by the drift limit. This exemplary embodiment or anotherexemplary embodiment may further provide wherein creating the pluralityof coherence domains further comprises: calculating a number of signalsamples to be taken per bin based on the smaller of the size constraintin the time domain and the size constraint in the frequency domain; andsampling the signal. This exemplary embodiment or another exemplaryembodiment may further provide wherein establishing the plurality of FFTbins further comprises: selecting an FFT size closest to the number ofsamples per bin; and determining the number of bins per coherencedomain.

In another aspect, an exemplary embodiment of the present disclosure mayprovide a method of secure communications within an environmentcomprising: generating a communications signal with at least a portionof the signal being known and at least a portion of the signal beingunknown; transmitting the communications signal from a first platformvia at least one antenna in operable communication with at least onetransmitter; receiving the communications signal with a second platformvia at least one antenna in operable communication with at least onereceiver; creating a plurality of coherence domains using time andfrequency constraints to accept maximum uncompensated Doppler values;establishing a plurality of FFT bins in each of plurality of coherencedomains; estimating the synchronization values of the communicationssignal within the coherence domains based on the known portion of thecommunications signal; estimating the synchronization values of thereceived signal between coherence domains based the known portion of thecommunications signal; and updating the synchronization values of thecommunications signal on an ongoing and real time basis using theestimated synchronization values from the detected signal from a postprocessing feedback loop. This exemplary embodiment or another exemplaryembodiment may further provide wherein estimating the synchronizationvalues of the received signal within coherence domains is performedusing coherent processing techniques. This exemplary embodiment oranother exemplary embodiment may further provide wherein estimating thesynchronization values of the received signal between coherence domainsis performed using non-coherent processing techniques. This exemplaryembodiment or another exemplary embodiment may further provide whereincreating the plurality of coherence domains further comprises:calculating a size constraint in the time domain based the highestDoppler frequency of concern determined from a maximum uncompensatedvelocity between the first platform and the second platform. Thisexemplary embodiment or another exemplary embodiment may further providewherein the value of the size constraint in the time domain isapproximately one quarter of a period of the highest Doppler frequencyof concern. This exemplary embodiment or another exemplary embodimentmay further provide wherein creating the plurality of coherence domainsfurther comprises: calculating a size constraint in the frequency domainbased on a maximum Doppler frequency offset that is less than the FFTbin size determined by a drift limit of less than 180°; and calculatingthe number of coherence domains required to keep a Doppler skew below10°. This exemplary embodiment or another exemplary embodiment mayfurther provide wherein the size constraint in the frequency domain isapproximately one tenth of the FFT bin size determined by the driftlimit. This exemplary embodiment or another exemplary embodiment mayfurther provide wherein creating the plurality of coherence domainsfurther comprises: calculating a number of signal samples to be takenper bin based on the smaller of the size constraint in the time domainand the size constraint in the frequency domain; and sampling thesignal. This exemplary embodiment or another exemplary embodiment mayfurther provide wherein establishing the plurality of FFT bins furthercomprises: selecting an FFT size closest to the number of samples perbin; and determining the number of bins per coherence domain.

In another aspect, an exemplary embodiment of the present disclosure mayprovide a system comprising: at least one antenna; at least one receiverin operable communication with the at least one antenna, the at leastone antenna and at least one receiver operable to receive and transmitelectromagnetic signals; at least one processor capable of executinglogical functions in communication with the at least one antenna and atleast one receiver; and at least one non-transitory computer readablestorage medium having instructions encoded thereon that, when executedby the processor, implements operations to generate a waveform, theinstructions including: receive a signal via the at least one antennaand the at least one receiver, the signal having a known preamble andwherein the data used in generating the signal are not known except forthe data symbols in the preamble; create a plurality of coherencedomains using time and frequency constraints to accept maximumuncompensated Doppler values; establish a plurality of FFT bins in eachof plurality of coherence domains; perform a series of bulk correlationson the received signal by multiplying the received signals with areference signal derived from the known preamble in the frequency domainand applying an inverse fast Fourier transform (FFT) to the result toestimate the correlation values at points in time; detect and isolatethe preamble of the received signal; estimate the synchronization valuesof the received signal within the coherence domains based on the knownpreamble; estimate the synchronization values of the received signalbetween coherence domains based on the known preamble; and update thesynchronization values of the received signal on an ongoing and realtime basis using the estimated synchronization values from the detectedsignal from a post processing feedback loop.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Sample embodiments of the present disclosure are set forth in thefollowing description, are shown in the drawings and are particularlyand distinctly pointed out and set forth in the appended claims.

FIG. 1 is a schematic view of a radio system according to one aspect ofthe present disclosure.

FIG. 2 is a block diagram view of a cognitive radio system according toone aspect of the present disclosure.

FIG. 3 is a block diagram view of a LPx communications system withmodular open system architecture according to one aspect of the presentdisclosure.

FIG. 4 is a block diagram view of the physical layer of the LPxcommunications system of FIG. 3 according to one aspect of the presentdisclosure.

FIG. 5 is a block diagram view of an LPx chaotic physical layeraccording to one aspect of the present disclosure.

FIG. 6 is an exemplary flow chart illustrating a method of generating achaotic waveform signal according to one aspect of the presentdisclosure.

FIG. 7 is an exemplary flow chart illustrating a method of generating achaotic carrier for the chaotic waveform from the process depicted inFIG. 6 according to one aspect of the present disclosure.

FIG. 8 is a block diagram view of a receiving unit of an LPxcommunications system according to one aspect of the present disclosure.

FIG. 9 is an exemplary graphical view of a series of coherence domainsaccording to one aspect of the present disclosure.

FIG. 10 is a second exemplary graphical view of the series of coherencedomains of FIG. 9 according to one aspect of the present disclosure.

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION

The methods described herein relate to generation and to synchronizationof a waveform for low probability of detection (LPD) and low probabilityof exploitation (LPE) communications. These methods are described withreference to an example of a radio system as used with a platform;however it will be understood that these methods may be employed withvarious different radio systems and/or different platforms, as discussedfurther below. These methods represent an improvement over prior LPD/LPEcommunications signals in that they enhance and improve both generationand synchronization of a waveform over prior art solutions. Further,these methods increase the operating efficiency and performance of radiosystems while further allowing for improvements in LPD/LPEcommunications through reduced probability of detection, as discussedherein. Accordingly, while, in one instance, these methods may beimplemented through the use of existing and/or legacy systems, it willbe understood that they represent improvements to the operation andfunctioning thereof.

As discussed further herein, while the primary focus of this method isLPD/LPE military communications, the same methods may be used by analogyin commercial applications for underlay communication. If there areexisting communications in a band, and the receivers of those existingcommunications have a certain tolerable signal-to-noise ratio (SNR) orspectral flux density (SFD), the methods described here could be used toestablish additional communications that do not interfere unacceptablywith the existing communications. Those existing communication could beof either a commercial or military nature. The new communicationscapability established may or may not be required to be LPD/LPI. Howeverthey would have the additional requirement of not interfering withexisting communications (or other RF functions such as radar) in a givenband.

With reference to FIG. 1 , a generalized radio system is shown andgenerally indicated at reference 10. As shown in FIG. 1 , a basicrepresentation of radio system 10 may include one or more antennas 12,one or more transceivers 14, and one or more processors 16. As depicted,radio system 10 may be installed on a platform 18, as discussed furtherbelow. As a whole, radio system 10 may be any suitable radio system 10and may include legacy assets therein or may be modified as dictated bythe desired implementation. According to one aspect, system 10 may be aradio system 10 as described in U.S. patent application Ser. No.16/804,104, which has been incorporated herein by the reference above.

As used and understood herein, antenna 12 may be an antenna array, whichmay include one or more antennas 12 in any configuration and may beinstalled in any position on platform 18. Antenna 12 may be a monopole,dipole, directional, or omni-directional antennas 12, or may be anycombination thereof. Antenna(s) 12 may be arranged in any desiredconfiguration appropriate for the installation conditions, includingexisting legacy configurations on a platform 18 as dictated by thespecific installation parameters and the type of platform 18 used. Itwill be understood that one particular antenna 12 arrangement may workbetter for a particular platform 18 with another antenna 12 arrangementbeing better suited for a different platform 18. By way of onenon-limiting example, where platform 18 is an aircraft, it may be bettersuited for a particular antenna 12 arrangement(s) while a land-based orsea-based vehicle platform 18 may find advantages with a differentantenna 12 arrangement(s). According to one aspect, antennas 12 maydiffer in type by function. For example, as discussed further below,antennas 12 utilized for receiving a signal may be of one type whileantennas 12 used for transmitting a signal may be of the same or of adifferent type.

Antenna(s) 12 may include receiving antennas and transmitting antennas,which may be operable to broadcast and/or receive radio signals directedto and/or from the platform 18 as discussed further herein. According toone aspect, the antennas 12 may be in communication with transceiver 14and/or processor 16 such that signals received by antennas 12 may becommunicated to transceiver 14 and/or processor 16, as discussed furtherherein. Similarly, signals being generated by processor 16 and/ortransceiver 14 may be communicated to and broadcast out from platform 18via antennas 12. Where multiple antennas 12 are utilized, they may beconfigured such that all antennas 12 may be of the same type (e.g. dualband, directional, omni-directional, etc. . . . ) or may be configuredsuch that multiple types of antennas 12 may be employed together or inclose proximity with each other. According to one example, a radiosystem 10 may have one antenna 12 or antenna 12 array configured for afirst signal, such as a dual band signal while also having antenna(s) 12configured for a second signal, such as a directional signal.

According to one aspect, system 10 may include full multi-inputmulti-output (MIMO) functionality. System 10 may therefore furtherinclude a dedicated exciter and/or power amplifier for each antenna 12provided therein. According to another aspect, as discussed previouslyherein, system 10 may include one or more antenna arrays in place of oneor more of the antennas, which may be adaptive antenna arrays.Alternatively, adaptive arrays may be provided in line with one or moreantennas 12, as dictated by the desired implementation.

The one or more transceivers 14 may be operable to both transmit and/orreceive radio waves via the antennas 12 and may include any type oftransmitter, including but not limited to, communications transmitters,radar transmitters, signal jamming transmitters, or the like. Accordingto one aspect, transceiver 14 may include more than one type oftransmitter. Transceiver 14 may further include any type of receiver,including but not limited to, radio receivers, global navigationreceivers, very high frequency omni-directional range (VOR) receivers,or the like. According to another aspect, transceiver 14 may includemore than one type of receiver.

Transceiver 14 may be operable to transmit and/or receiveelectromagnetic signals via the antennas 12, as discussed furtherherein. According to another aspect, transceiver 14 may be both atransmitter and a receiver realized as separate assets within the radiosystem 10 as dictated by the desired implementation parameters.

Transceiver 14 may be in communication with one or more processors 16through a wired or wireless connection, which may be a directconnection, a direct serial connection, or a wireless connection.According to another aspect, transceiver 14 may be in communication withprocessor 16 through intermediate components that may be included orotherwise utilized by radio system 10 according to the desiredimplementation. For example, transceiver 14 may be in communication withprocessor 16 by way of one or more frequency converters or the like.

The one or more processors 16 may be one or more computer processors,logics or series of logics, including or otherwise in communication withone or more non-transitory storage mediums. The processor 16 may be adigital processor capable of carrying out and executing a set ofinstructions or processes encoded thereon as further discussed herein.According to one aspect, processor 16 may be operationally connected toother components of radio system 10 as discussed further herein.According to another aspect, processor 16 may be remote from other radiosystem 10 components and may be in wired, wireless, or in any suitablecombination of wired and wireless communication therewith. Theconnectivity and communication between other radio system 10 componentsand processor 16 may therefore vary, depending upon the desiredimplementation and installation parameters of radio system 10 asdiscussed herein.

Transceiver 14 and/or processor 16 may further be in communication withother systems onboard the platform 18 such that relevant data may becommunicated there between. For example, where platform 18 is anaircraft, onboard flight systems may relay the data to the transceiver14 and/or processor 16 such as heading, altitude, flight speed,geolocation, and the like. Similarly, transceiver 14 and/or processor 16may communicate data regarding detected signals and the like to theplatform 18, including to the operator or operators thereof. Asdiscussed further below, communication between the platform 18 and theradio system 10 may allow specific actions to be taken by platform 18.For example, where platform 18 is an unmanned aircraft such as a drone,platform 18 may take automated actions such as steering towards or awayfrom a specific location to best fit the needs of radio system 10 andany communications being sent therefrom or received thereto. Whereplatform 18 is a manned platform, such as a manned aircraft, platform 18may take similar automated responsive action or may alternatively allowthe operator or pilot of the platform 18 to choose whether or not toemploy responsive actions.

According to one aspect, transceiver 14 and processor 16 may be separateassets within radio system 10. According to another aspect, transceiver14 and processor may be a single, integrated unit within radio system10. It will be therefore understood that the illustration of radiosystem 10 as shown in FIG. 1 is an exemplary view and not a limitingview thereof.

Platform 18 may be any suitable unit which may be capable of operatingin a congested environment, a contested environment, or a hostileenvironment and further capable of utilizing radio communications.According to one aspect, platform 18 may include manned portable radiosystems 10 carried by individual troops. According to another aspect,platform 18 may be ground vehicles, sea-based vehicles, aircraft,including manned and unmanned, and the like carrying radio system 10thereon or therewith. According to another aspect, platform 18 may be amunition, rocket, or other propelled vehicle. According to anotheraspect, platform 18 may be a remotely operated vehicle. As used herein,platform 18 is illustrated as an aircraft (such as in FIG. 1 ); however,the examples and description provided herein will be understood to beequally applicable across all versions of platform 18 as dictated by thedesired implementation, unless specifically stated otherwise.

As further used herein, a congested environment is contemplated to be anenvironment with a high utilization of the electromagnetic spectrum andmay include civilian areas or areas that are not considered to include athreat. A contested environment is contemplated to be an environmentwhere adverse or opposing units are operating, scanning, monitoring orotherwise present within an area of operations. A hostile environment,as used herein, is contemplated to be an environment that is contestedin a manner such that a threat is posed to platform 18 as it operates inthe area of operations. Accordingly, a hostile environment is acontested environment; however, a contested environment may not alwaysbe hostile. Both contested and hostile environments may be congested ornot congested depending upon the utilization of the electromagneticspectrum therein.

The area of operations may be referred to herein and collectively as the“theater” and will be understood to include both contested and hostileenvironments, unless specifically stated otherwise. An adverse unit,enemy, or enemy unit, as used herein, are all contemplated to be anymember or unit of an opposing force operating in the same environment,including, but not limited to persons, vehicles, stationaryinstallations, satellites, or the like.

With reference to FIG. 2 , the architecture of one embodiment of a radiosystem 10 according to the present disclosure is shown having a strategyoptimizer component 20, a mitigation control plan component 22, and aninterference recognizer component 24 which are shown as part of a blockdiagram relating to processor 16 and/or transceiver 14, as discussedherein. While each of these components, namely, strategy optimizer 20,mitigation control plan component 22 and interference recognizer 24 mayvary depending upon the desired implementation, they will be understoodto be described herein with reference to one embodiment and not as alimiting example of radio system 10 inasmuch as other components may beutilized or omitted as dictated by the desired implementation. Theexemplary radio system 10 of FIG. 2 is shown and described as a“cognitive” radio system 10 in that the inclusion of the strategyoptimizing component 20 may allow for system 10 to “reason” over a setof criteria (implemented as policy component 30) which may oversee orotherwise place proper limitations on the strategies and responsesproduced and utilized by radio system 10, as discussed below, tominimize the probability of detection, exploitation, or the like ofsystem 10. According to another aspect, other, (non-cognitive) systemsmay be employed with the methods described herein.

According to the example set forth in FIG. 2 , strategy optimizingcomponent 20 (also referred to herein as strategy optimizer 20) of radiosystem 10 may further include a long term response engine 26 and a rapidresponse engine 28, which may be in communication with one another viaone or more data connections 32 to allow strategies to be developed bothutilizing long term data and utilizing time data collected duringoperation and use of radio system 10. Strategy optimizer 20, or morespecifically, long term and short term response engines 26, 28 may befurther connected to a mitigation control plan component 22 through oneor more data connections 32. Both long term response engine 26 and rapidresponse engine 28 may be further connected to interference recognizer24 by a performance feedback loop 34, which may facilitate real timeadjustments or modifications to the radio system 10 during operation.

A variety of strategy optimizing components 20 can be used within thissystem 10. For example, the long term and short term response engines26, 28 may be existing designs that may be adapted for use in strategyoptimizing component 20. According to another aspect, long term andshort term response engines 26, 28 may be adapted from existing designsor may be designed and built for a specific implementation, according tothe desired parameters of such an implementation.

According to another aspect, other forms of machine intelligenceincluding, but not limited to, a simple state machine could also be usedin place of the strategy optimizer 20, or in place of specificcomponents thereof. By way of one non-limiting example, a set ofcomputations to determine allowed power transmission based on policy 30and observables, such as the current location of platform 18, may beapplied. This policy 30 may be based on known link length, for example,and an allowed data rate could be determined and applied with a simplestate machine, or the like.

Strategy optimizer 20, or more particularly, long term and short termresponse engines 26, 28, may accept “observables” as provided by theinterference recognizer 24 and interface to “controllables” such asprovided by the mitigation component 22 to improve overall performanceof the system 10, as discussed further herein.

As mentioned previously herein, it will be understood that thearchitecture of system 10 as depicted in FIG. 2 may be modifiedaccording to the desired implementation; however, the architecture mayalso operate similarly to legacy systems in that various componentsprovided therein, including one or more of strategy optimizer 20,mitigation control plan component 22, and/or interference recognizer 24may be legacy assets and/or may be used in connection with other legacyassets as dictated by the desired implementation.

As used herein to this point, LPD and LPE communications are defined bythe waveform configuration based on mission needs, specifically, thewaveform needs to have a low probability of detection or ofexploitation; however, there are other waveform considerations that maydiffer according to specific mission parameters and/or missionobjectives. These mission specific waveform configurations may becollectively thought of as a family of waveforms, with each configuredfor a different set of needs to meet a different set of goals. By way ofsome non-limiting examples, in addition to LPD and LPE, a mission maydesire or require waveforms with a low probability of intercept,geolocation, recognition, jamming, spoofing, and/or breaking, or thelike. Collectively, these waveform considerations and configurations mayhereinafter be referred to as LPx, wherein x is understood to representa variable waveform that can be adapted for multiple considerations.Accordingly, it will be further understood that LPx communications mayinclude one or more of such waveform configurations, unless specificallystated otherwise.

With reference to FIG. 3 , radio system 10 may be utilized as acomponent in a larger LPx communications system with modular open systemarchitecture (MOSA), shown and generally referenced at 36 in FIG. 3 .The architecture of LPx system 36 may include a gateway 38, aninter-network router (INR) 40, and a modular waveform stack 42 which mayallow LPx system 36 to adapt to varying mission needs, including theaccommodation and use of multiple differently configured waveforms,including non-LPx waveforms where desired. Cognition, as discussed abovewith respect to radio system 10, may be applied throughout LPx system,as dictated by the desired implementation through the inclusion ofsystem 10 and/or similar cognitive components throughout the gateway 38,INR 40, and/or one or more waveform stacks 42.

Gateway 38 may be an intelligent gateway 38 which may be operable toprovide a control interface to applications, mission managers, andexternal situational awareness components. Gateway may further beoperable to negotiate quality of service between applications andnetwork, ensure that communications components conform to missionpolicy, provide for information discovery and warehousing, supporttranscoding, and encrypting of information, as needed. Gateway 38 mayuse cognitive technologies to learn and adapt, and coordinate with othercognitive components in this architecture of the LPx system 36. Gateway38 may be further operable to perform other desired functions asdictated by the specific implementation and/or specific mission needs.Accordingly, references to gateway 38 will therefore be understood toinclude any necessary components and elements therein to accomplishthese tasks, or to accomplish any additional tasks assigned theretoaccording to the desired implementation.

INR 40 may be an intelligent inter-network router 40 which may beoperable to provide heterogeneous MANET routing across availablewaveforms. INR 40 may be further operable to provide flow control andtopology management, perform adaptation, and routing as needed across awaveform. INR 40 may coordinate activities closely to gateway 38 and mayaccept flow down of policy and situational awareness from gateway 38interface (represented in FIG. 3 as the dual sided arrow between gateway38 and INR 40). Similar to gateway 38, INR 40 may use cognitivetechnologies to coordinate between nodes and waveforms within the LPxsystem 36.

Waveform stack 42 may be a traditional open systems interconnectionstack with a cognition component, such as radio management system 10 onthe control and management plane. Other control and management planecomponents of waveform stack 42 may include a situational awarenesscomponent 44, while other components, including, but not necessarilylimited to, networking 46, link/MAC 48, physical (PHY) layer 50, and RFincluding antenna 52 components may be layered on the data plane and maybe red side or black side components, as dictated by the specificimplementation. This may allow waveform stack 42 to map to a wide arrayof radio architecture, and may further provide that any layer may beremoved and used within different waveform stacks, including withinexisting waveform stacks.

Waveform stack 42 may be operable to provide a “link” for the INR 40.Within a waveform, the waveform stack 42 may provide routing betweenwaveform instances, scheduling of media access, framing, modulation,etc. Waveform stack 42 may likewise include cognitive learning andadaptation features that account for local sensing and situationalawareness provided by other components, such as the situationalawareness component 44. The waveform stack 42 is structured toaccommodate integration of cryptographic functions (red/black) and mayfurther allow for more limited awareness of the black side andintegration of “inter-waveform” functionality.

With reference to FIG. 4 , the PHY layer 50 may be functionallydecomposed into several interdependent components including, but notlimited to, framing 54, control signaling 56, packetization 58, errorcorrection 60, interleaving 62, synchronization 64, modulation 66, andcarrier generation 68. The PHY layer 50 may further include extensions70 to one or more of the components which may be or include long term:multi-user (MU), multiple input-multiple output (MIMO), and/ordirectional networking (DN) technologies. As discussed further herein,the primary focus for LPx constraints may include the synchronization64, modulation 66, and/or carrier generation 68 components. Thesecomponents will be collectively referred to as LPx components 72, whichwill be understood to include all three identified components, unlessspecifically stated otherwise.

These components of PHY layer 50 are to be understood as exemplarycomponents and may vary depending upon the desired implementation.According to one aspect, additional components may be included with thePHY layer 50. Alternatively, one or more of the PHY layer 50 componentsdiscussed herein may be omitted or replaced with other similarcomponents, as desired

As mentioned previously herein, it will be understood that thearchitecture of systems 10 and 36 as depicted and described may bemodified according to the desired implementation; however, thearchitecture may also operate similarly to legacy systems in thatvarious components provided therein may be legacy assets and/or may beused in connection with other legacy assets as dictated by the desiredimplementation.

Having thus generally described LPx system 36, the methods of generationand synchronization of a waveform may best be understood throughdiscussion of these methods themselves. Accordingly, the elements andcomponents of LPx system 36 will now be discussed with respect to theiroperation and function to further illustrate and understand thedisclosed methods.

As a general concept, the goal for the disclosed waveform is to generatea LPx waveform that is not just LPD, but accounts for other constraintssuch as low probability of geolocation (LPG). Put another way, while itis extremely important to avoid detection, it is equally important inmany situations to avoid the ability of adversarial units being able togeo-locate platform 18 using the transmitted signal. Put another way, ifan adversarial force can't detect the presence of platform 18, theycan't geo-locate the platform 18; however, if the adverse force doesdetect transmissions from platform 18, it is important that they cannotthen use the detection of the transmissions to geo-locate platform 18.The architecture of LPx system 36 may allow for LPD or LPG waveforms (aswell as other LPx waveforms) via flux density limits. It is thereforedesirable to have the LPx characteristics of the disclosed waveform toappear as noise to adversarial detectors. Some high level ways to goabout this may include transmission of two or more signals that spoofand/or cover each other, adjusting individual time and/or frequencycomponents to appear to have different delays or different Dopplerprofiles, and/or limiting the total power and/or data rate of thetransmitted signal based on energy detection thresholds based on thenoise floor of the detector. Many of these will be discussed below.

There are many instances and scenarios in which a radio orcommunications equipment may be desirable to be operated in anyenvironment, including contested or hostile environments. One suchnon-limiting example involves battlefield communication and/orcoordination of units in a hostile environment. Accordingly, while it isunderstood that there are multiple applications for an LPx system 36and/or radio system 10, the disclosure herein is described withcontemplation towards such scenarios involving battlefieldcommunications for the express purpose of simplicity and clarity in thedisclosure. It will be therefore further understood that these are notlimiting examples of use and operation the disclosed systems and methodsbut exemplary operations and use thereof.

Again, as previously described above, radio signals may be exploited byan adversarial force in a number of ways. First, radio signals may bedetected by an enemy receiver and may be utilized to determineinformation as basic as the existence of units operating in a certainarea, to more complicated information as to the type, size, direction,heading, velocity, and/or current location of these communicating units.Further, even secured communications run the risk of being interceptedif the signals are not also hidden or otherwise made difficult to detector exploit in the first place.

Therefore, it is desirable to maintain these communications in a mannerthat provides LPx qualities depending upon the desired level of secrecyneeded in that particular operation and/or for that particular unit.Despite the fact that enemies may be able to detect and/or utilize andexploit electromagnetic signals such as radio waves to their advantage,radio silence in a contested or hostile environment such as abattlefield is often unfeasible as there are certain informationexchange requirements (“IER(s)”) that must be communicated amongst unitsoperating therein. For example, these IERs may include such informationas targeting, information regarding friendly force location (otherwiseknown as blue force tracking), battle plans, action changes or reactivemaneuvers, recon information, and the like. Often this involves twolevels of communication. For example, a group of soldiers operating in ahostile area may be close together and thus may not require muchbandwidth or power to talk amongst themselves, but there is a need toeffectively communicate as a group to other units or persons who may belocated further away, such as battlefield control or a missioncommander. These remote units/persons may be located many miles or morefrom the troops operating and communicating the in environment.Communications to that effect require much higher power and increasedbandwidth. By way of a simplified and non-limiting example, twoindividuals next to each other may communicate effectively bywhispering, but to communicate with a person on the other side of alarge field, they may need to shout. Relating this example to radiocommunications, the greater the distance separating the communicatingparties, the “louder” the parties need to talk to effectivelycommunicate. The tradeoff is that when power and/or bandwidth needsincrease, the likelihood of a signal being detected and/or exploited byenemy units increases as well.

The present disclosure addresses ways of generating and receiving LPxcommunications that exploit inherent difficulties in receivertechnologies to reduce the likelihood of signal detection andexploitation. One such application can involve the use of SFD limits toallow LPx system 36 to approximate SNR walls for known receivers whichmay then permit communications using signals that fall below the SNRwall. Thus, a signal operating below the SNR wall is likely to be viewedas noise and readily disregarded.

Once the SNR that a receiver requires to detect a signal isapproximated, the receiver's noise figure and antenna gain can beobtained. With this information the SFD for any particular receiver maybe calculated. The SFD essentially stands for the power density that maybe used for communications without being detected by that receiver. Inother words, SNR walls, noise, and antenna gain can be translated intoSFD limits for each receiver and may therefore allow for LPx waveformsto operate below these limits while maintaining a high throughput andwithout curtailing battlefield communications.

Accordingly, the parameters of a chaotic LPx PHY 50 become important tomaintain a signal status below the SNR wall and SFD limits. Accordingly,the parameters for an LPx PHY 50 include slot size, a preamble, datapulses, a start jitter, pulse gap, pulse width, allocated frequency,occupied bandwidth, frequency hopping, direct sequence minimum shiftkeying (DS-MSK), frequency shift keying (FSK), chaotic carrier binaryphase shift keying (CC-BPSK), and chaotic carrier cyclic shift keying(CC-CSK). Using these parameters in combination (or variations ofcombinations of parameters) allows for a standard set of profiles to bedetermined on a pre-mission basis for each subnet. Each of theseparameters will now be briefly discussed.

Slot Size—LPx may allow for a variable slot size, where slots are bestunderstood as transmission opportunities and the waveform canaccommodate multiple transmissions per each slot and also foroverlapping slots. According to one example, an LPx waveform could havea slot size of 50 msec even though an existing overlaid waveform used 10msec slots. LPx waveforms may further support simultaneous transmissionin multiple overlaid slots.

Preamble—A number of dedicated synchronization pulses may be included atthe front of the waveform. The only difference between sync and datapulses is that sync pulses have a known data value that facilitate timeand frequency synchronization of the waveform as well as channelestimation. The preamble is optional, and preamble-less operation issupported by the LPx PHY 50.

Data—The LPx waveform supports a variable number of data pulses perslot. This may allow for different pulse structure (degrees of LPD),propagation delays, etc. According to one aspect, the data pulses may bedefined to match with existing coding and interleaving schemes forexisting LPx and non-LPx waveforms.

Start Jitter—To limit the impact of cognitive jammers, a uniformpseudo-randomly varied transmission start time (with max offset fromslot boundary of a maximum jitter) may be used to avoid a systemlearning the slot structure. Long term cognitive jammers and signalsintelligence (SIGINT) may be further confused by the overlapping ofmultiple slots.

Pulse Gap—A variable gap between pulses is permitted with the LPxwaveform. A minimum fixed gap (which can be set to 0) may be determinedwith an additional uniform pseudorandomly generated variable gap portionwith a max value determined by variance (as discussed below). The gapmust be an integer number of samples.

Pulse Width—A variable pulse width is permitted in the LPx waveform. Thewidth must be an integer number of samples. The minimum fixed width maybe determined with an additional uniform pseudorandomly generatedvariable with portion with a max determined by variance.

Allocated Frequency—For purposes of the present disclosure, a singlecontiguous frequency band is assumed; however, the LPx waveform mayaccommodate keep outs, other spectrum management constraints, andawareness if desired according to the specific implementation.

Occupied Bandwidth—The “instantaneous” bandwidth of the pulse is tunablefrom a full allocated frequency bandwidth down to an arbitrary valueconsistent with pulse length.

Frequency Hops—If enabled, the LPX waveform is capable to automaticallyhop over the allocated bandwidth in uniform pseudorandomly generatedincrements consistent with the modulation and pulse width optionsselected.

DS-MSK—When combined with Frequency Hopping, DS-MSK can mimic mostpopular forms of LPD and anti-jamming communications available today.The DS-MSK as contemplated by the present disclosure may utilize arandom spreading code with length determined by the pulse widthparameters. The DS code may be modulated with cyclic shift keying (CSK)based on the symbol order. Thus, if the symbol order is set to 3,2{circumflex over ( )}3 (8) cyclic shifts would be used to modulate dataonto the DS carrier.

FSK—FSK mode does not require a linear PA, and is the most difficult togeo-locate. For each pulse, a single frequency carrier with uniformpseudorandomly generated start phase may be transmitted. The frequencytransmitted may be determined by the parameter symbol order. If thesymbol order was set to 3, one of 8 different frequencies would be usedfor each symbol. The tones may be equally spread so as to fully occupythe occupied bandwidth defined for the signal. When frequency hopping isenabled, the whole tone set is hopped across the available spectrum.This mode may use M-ary orthogonal modulation frequency shift keying tomodulate a carrier.

CC-BPSK—This modulation is the most LPD of the modulations available asit has the least features. The chaotic carrier is a complex sequence ofGaussian variables of zero mean and unit variance. The number of samplesin the sequence may be determined by the pulse width. The sequence isfiltered to the occupied bandwidth, and then multiplied by +1 or −1(Phase shift keyed) depending on the data value. For purposes of thisdisclosure, a symbol order of 1 is currently supported and coherentdemodulation may be assumed.

CC-CSK—This modulation is less LPD than CC-BPSK but does not requirecoherent demodulation. The chaotic carrier is generated the same was asfor CC-BPSK, but instead CSK (same as for DS-MSK) may be used tomodulate the carrier.

According to one non-limiting example, a rateless CC-BPSK profile mayhave the following parameters:

-   -   Slot Size—7.8125E-3 (seconds)    -   Preamble—36 (pulses)    -   Data—2048 (pulses)    -   Start Jitter—1.0 E-4 (seconds)    -   Pulse Gap=[0,0] (samples)    -   Pulse Width=[896, 256] (samples)    -   Allocated Frequency=250E6 (Hz)    -   Occupied Bandwidth=250E6 (Hz)    -   Frequency Hopping=0 (OFF)    -   DS-MSK=[0,1] (Chaotic Carrier, BPSK)

With reference now to FIG. 5 , an exemplary block diagram of an LPxchaotic PHY 50 is shown having a data link layer 74 connecting to boththe transmit plane 76 and the receiving plane 78. The transmit plane 76may further include an error correction coding module 80, a syncinsertion module 82, and a chaotic spreading module 84 between data linklayer 74 and one or more transmit antennas 12. The receiving plane 76may further include a de-spreading module 86, a sync detection module88, a synchronization module 90, and an error correction decoding module92 between one or more receiving antennas 12 and the data link layer 74.The LPx PHY 50 may further include a key store 94 and frame keygenerator 96.

As seen throughout FIG. 5 are transmission security (TRANSEC) bits whichmay be generated from key store 94 and frame key generator 96, asdiscussed below, and are applied at the synchronization and spreadingmodules 82, 84, 86, 88, and/or 90 in both the transmitting and receivingplanes 76 and 78. Chaotic waveforms, as contemplated here, require alarge number of random bits for each information bit. These random bitsare the TRANSEC coded bits. One concern raised by this is the potentialfor key exhaustion, which is where a TRANSEC key, which allows forcoding and decoding a signal, is used with enough bits as to reach thepoint of exhaustion. This point is where the key is no longer useful asenough TRANSEC bits have been exposed that it is considered vulnerableto exploitation. For TRANSEC key generation, it is common to use arandom bit generator, such as AES-256, although any suitable random bitgenerator could be employed. Assuming frame keys are changed once perframe or slot, an AES-256 key would have approximately 2{circumflex over( )}30 bits (i.e. uses) before it is exhausted. Assuming normal use witha chaotic waveform, one key could last on average four to six hoursbefore reaching the point of exhaustion. Accordingly, for missions thatare expected to come close to or exceed this time frame, additional keysare required. One solution may include using multiple random bitgenerators to produce session keys which can then be used to produce theframe keys. From there, the TRANSEC may be generated and applied to theoutgoing or the incoming signals. According to one example, a firstAES-256 (or any other suitable random bit generator) may generate one ormore session keys which may be stored in key store 94. One session keyat a time may then be delivered to the frame key generator 96 which maygenerate the frame key using a second AES-256 (or suitable alternative)random bit generator to produce the TRANSEC. Once a session key isexhausted, a new session key can be delivered from the key store 94 tothe frame key generator 96 to allow continued operation.

If operating the LPx waveform below the SNR wall, for example in a rangefrom (−20) to (−30) dB, enemy receivers will not be able to detect thesignal, but with the correct TRANSEC, the signal can still be receivedand processed by the target unit. Further, the waveform may beconfigured according to the parameters discussed herein to optimize thesignal according to the environment being operated in. For example,where a platform 18 is operating in an environment with a receiver thatis a known distance away (the distance to intercept—Di) and the distancethe platform 18 needs to transmit its signal (distance ofcommunication—Dc), the waveform parameters may be adjusted or optimizedto provide the Dc:Di ratio needed at that moment. LPx system 36 mayaccomplish this in real time through the use of cognitive reasoning, asdiscussed previously herein.

The chaotic waveforms generated by the LPx system 36 may be compatiblewith a variety of forward error correction (FEC) protocols, including,but not limited to, low density parity check (LDPC), Reed Solomon (RS),and repeat codes. Further, the disclosed method (discussed below) iscontemplated to utilize a set of unmodulated pulses, for synchronizationwhich may include a pilotless tracking loop to maintain synchronization,or may alternatively leave some Fast Fourier Transform (FFT) binsunmodulated as pilots for tracking.

The PHY 50 is thus chaotic as the carrier appears to be random noise,but is actually known by the transmitter (i.e. platform 18) and thereceiver (e.g. the communications target platform) based on the framekey. Further, each pulse of the waveform may be bi-phase modulated (orbi-modulated) by flipping the phase 180° depending on whether a digital‘1’ or ‘0’ is transmitted.

The PHY 50 may also generate transmission pulses having varying numbersof samples in each pulse. This permits the PHY 50 to be consideredrateless. According to one example, a particular variance in pulse sizemay be chosen using a random number to determine the size of each pulse.For this example, assume a 10% variance of the average pulse size isused. Then, each pulse may vary in size (i.e. in number of samples)within that 10% variance which may result in a rateless waveform that isdifficult to detect by rate detectors. The various sized pulses in apulse train are able to be well bounded through the utilization of asimple algorithm. Specifically, if the current pulse is set to thesmallest permissible pulse size (minus 10% from the average), a uniformrandom number may be generated and used to increase the intendedvariance. Then, the nominal pulse size for this first pulse may be equalto: the min. pulse size+variance/2. The random number may then be addedto the current pulse size to get the final pulse size.

The next pulse is then set to the minimum pulse size and the variance ofthe previous pulse is subtracted therefrom. Then, the variance/2 isadded to that result along with a new random number, up to the intendedvariance to get the size of the second pulse. This process may then berepeated for each subsequent pulse to keep the pulse train well bounded.

Having thus provided an overview of the waveform and of the methods ofwaveform generation and synchronization, the specifics of these methodswill now be discussed.

With reference to FIG. 6 , a process for generating a chaotic waveformwith LPx properties is shown and generally referenced as process 200.For each slot or frame in the waveform, there are a number ofinformation bits (info bits) which need to be accounted for. Theaccounting for info bits in each slot is shown as step 202 in process200. From there, if desired, the info bits may be encrypted (shown asstep 204) before applying FEC to each of the info bits using LDPC, RS,and/or repeat codes (or other suitable FEC protocols). The applicationof FEC to each info bit is shown at step 206.

Simultaneously, or in succession with, the application of FEC to theinfo bits, a pulse size and variance for that specific pulse may bedetermined and used, along with the aforementioned session and framekey, to generate the chaotic carrier for that pulse. The overallgeneration of the chaotic carrier is shown in process 200 as step 208and is understood to represent the generation of the chaotic carrier fora single, representative pulse. The method for generating the chaoticcarrier for a series of pulses is described below with reference toprocess 300 in FIG. 7 .

Once the FEC is applied, the info bits are now coded and may progressthrough process 200 to modulation with the chaotic carrier in step 210,which may be accomplished using the previously discuss bi-phasemodulation, or may be through any other suitable modulation protocol, asdictated by the desired implementation.

Once the coded info bits are modulated with the chaotic carrier, thesignal may be transmitted.

With reference now to FIG. 7 , a method of generating the chaoticcarrier for a series of pulses is shown and generally referenced asprocess 300. As discussed previously herein, process 300 is recognizedas the process of generating a chaotic carrier for a series of pulseswhich is performed during step 208 of process 200. Specifically, eachpulse of a pulse train will go through process 200, including step 208wherein a chaotic carrier is generated for each pulse. Every time achaotic carrier is generated, process 300 is employed during that step.

Accordingly, process 300 first begins with a determination of the pulsesize for the specific pulse within a pulse train. This determination isshown as step 302. Next in process 300 is a determination of which pulsewithin a pulse train is receiving a generated chaotic carrier. Moreparticularly, the determination is whether the present pulse is thefirst pulse of the pulse train. This determination is made as step 304.

If the pulse is determined to be the first pulse of the train in step304, the pulse size is calculated according to step 306 as the (minimumpulse size)+(random variance), as discussed previously herein. Fromthere, the chaotic carrier for the first pulse is generated and may thenbe modulated with the coded info bits in step 210 of process 200. Thereturn (e.g. sending) of the chaotic carrier for the first pulse in atrain is shown as step 308.

If the pulse is determined not to be the first pulse of the train instep 304, the pulse size is calculated according to step 310 as the(minimum pulse size)+(random variance)—(last variance)+(averagevariance)/2, as discussed previously herein. From there, the chaoticcarrier for each subsequent pulse is generated and may then be modulatedwith the coded info bits in step 210 of process 200. The return (e.g.sending) of the chaotic carrier for each subsequent pulse in the trainis shown as step 312 in process 300.

Processes 200 and 300 relate to a rateless chaotic physical layerutilizing pulse size variation methods which are useful in LPxcommunications for the reasons described above. It will be understoodthat additional waveform manipulations may likewise be useful orbeneficial in LPx communications.

According to one non-limiting example, a noise only channel with novariation of the sync or pulse size may be used to generate a chaoticcarrier which may start with uniformly sized pulses and uniform andRayleigh distributed random variables. The unform variables are used togenerate the phase values of the individual sine and cosine (I,Q)components of the chaotic charrier both of which are then multiplied bya common Reighley distributed variable. With uniform variables, thephases should have a uniform distribution and be equally likely;however, using the sine and cosign functions with the Rayleigh amplitudevariable will result in a Gaussian distribution (assuming two 8-bitnumbers i and q). This process may be performed utilizing FFT bins inthe frequency domain. A discrete Fourier transform (DFT) may be employedto define the chaotic pulse where the lowest DFT bins, and other bins ifdesired, may be set to zero. A Rayleigh amplitude variable multipliedtimes a complex sinusoid generated using a uniform phase distributionvariable can be used to populate non-zero bins. A time domain sequencecan be generated using an inverse DFT to generate the time domain puleswhich is then bi-phase modulated without varying the pulse width of thesignal. This approach may be further scaled to remain below the SNR wallgiven the specifics of a known detector for effective LPxcommunications.

According to another example, the noise only approach may be combinedwith variable pulse width to achieve similar or improved results.

Generating and transmitting an LPx signal is important and useful inbattlefield communications and in other instances where detection andexploitation of a transmitted signal is not desirable; however, thegeneration of such a signal requires that the communications target iscapable of receiving and understanding the data transmitted. Put anotherway, even if a “perfect” signal is transmitted that no detector canfind, if the target receiver cannot effectively receive and translatethat signal, it is useless in communications. Accordingly, LPx signalsnecessarily need to be properly synchronized with the target signalreceivers. While the synchronization techniques described herein arecontemplated for use with LPx signals, such as the wideband,featureless, rateless, chaotic waveform described above, it will beunderstood that these synchronization techniques may be used with andequally apply to other waveforms including LPx and non-LPx waveformsalike. For purposes of simplicity and clarity in this disclosure, thetechniques will be discussed with reference to the chaotic waveformstaught herein but it will be understood that the disclosure herein willapply to all waveforms and waveform types unless specifically statedotherwise.

An ideal chaotic signal is one that is wideband and is designed toappear as noise. Wideband, as used herein, is defined as a signal with abandwidth that is more than 10% of the center frequency, whileultra-wideband is a signal with a bandwidth greater than 25% of thecenter frequency. For example, a 250 MHz signal centered at 1 GHz wouldbe ultra-wideband under this definition.

If ideally designed and executed, a chaotic wideband or ultra-widebandsignal would appear as noise, which would essentially classify thesignal as arbitrary, which, as used herein is defined as a signal whereone or both of the coding and modulation used are not known. Counteringchannel effects of an arbitrary signal in synchronizing signals isdifficult at best and requires accounting for one or more of timingoffsets, frequency offsets, Doppler effects, and/or multipath. Thus, asolution that allows for synchronization of an arbitrary signal, such asa wideband or ultra-wideband chaotic signal, would work equally well tosynchronize other, less complex signals.

Typical synchronization techniques tend to fall into one of threefundamental approaches. First, there is a synchronization preambleand/or midamble which are a period of time where the transmitted signalis fully known. Next, there are approaches that utilize “pilot” tones,which are a set of frequencies (subbands) where the transmitted signalis again fully known. A third technique is the use of data aiding whichuses a modulated signal, but estimated data values are fed back to thereceiver so that the transmitted signal can be estimated. Most currentsystems use one or more of these approaches, with many using more thanone.

The present disclosure relates to a set of techniques that are based onthese three fundamental concepts and are tailored to an arbitrarysignal, such as the previously described chaotic waveform. This tailoredapproach may utilize receivers that presumes knowledge of transmittedsample values, but does not presume knowledge of underlying physicallayer operations such as modulation and/or coding.

According to one aspect, a present system of synchronization techniquesmay be operable using both directional and/or omni-directional antennas,would support time-domain multiple access (TDMA) systems while alsoconsidering frequency domain processing. Accordingly, disclosed hereinis a framework to handle synchronization for signals with an arbitrary,or unknown, modulation and coding scheme.

With reference to FIG. 8 , the framework of a receiver system is shownin a block diagram format having a synchronizer 98, a preamble detectionmodule 100, a delay module 102, and a channel estimator 104. Thisreceiver system is contemplated to be used within the PHY 50 framework(seen in FIG. 5 ) or separately as a standalone system which may bepaired (i.e. operable) with LPx system 36 or with any other suitabletransmitter system. According to one aspect, this receiver system mayfurther include a demodulator and decoder, such as the error correctiondecoder 92 of PHY 50, and a reference generator 106. The decoder may bedecoder 92 of PHY 50 or may alternatively be any other suitable decoderas dictated by the desired implementation. Reference generator 106 mayfurther form a feedback loop 108, as discussed below.

The receiver system of FIG. 8 may be understood to represent ageneralized example and is not a limiting example thereof. Instead, thegeneral concept illustrated is that an incoming signal, including apreamble when TDMA is employed, is received and simultaneously directedto the synchronizer 98, the preamble detection module 100, and the delaymodule 102. The preamble detection module 100 may then feed into thesynchronizer 98 while the delay module may feed to the channel estimator104 before ultimately feeding into the synchronizer 98. From there, thesynchronized signal may be delivered to the decoder 92 which may thendirect the decoded signal data to the data link layer (e.g. data linklayer 74 of PHY 50) while simultaneously being fed back into thereference generator 106 and back into the channel estimator 104 viafeedback loop 108.

Further according to this aspect, the received signal is delivered tothe synchronizer 98 in three conditions/ways. First, it is delivered asreceived directly from the receiving antenna. Second, the signal maypass through the preamble detection module 100 which may deliver aninitial timing offset, bulk frequency offset, Doppler, and/orequalization to the synchronizer. A third path may direct the signalthrough the delay module 102 and the channel estimator 104 which maydeliver a set of refined data relating to the timing offset, bulkfrequency offset, Doppler, and/or equalization which is further refinedthrough reference generator 106 and feedback loop 108 utilizing decodedsignal data to enhance the refinement of incoming signal pulses.

Each of the signal impairments, namely, the timing offset, bulkfrequency offset, Doppler, and/or equalization act to distort thetransmitted signal in some way, which may generally impact theamplitude, delay, or phase/frequency of a signal component. If the phaseof the signal is coherent, more powerful signal processing may be usedas the phase relationships are maintained by the signal after passingthrough the channel estimator 104, thus leading to improved detectionaccuracy. If the phase of the signal is non-coherent, less effective,but still viable solutions may be employed by focusing on the amplituderather than the phase to detect the signal.

According to one aspect, one way to address non-coherent signals is todeconstruct the signal into time frequency tiles, or bins. In doing so,phase relationships may be preserved to a higher level of fidelity whichmay allow coherent detection techniques to be used within each time;however, between differing tiles, these phase relationships are notpreserved so non-coherent detection may still be applied.

With reference then to FIG. 9 , the processing of a non-coherent signalmay utilize FFT to break the processing into several “coherence domains”which may accept maximum uncompensated Doppler. This approach assumesthe effects of multipath are constant or relatively constant within eachdomain which then allows for coherent processing within each domainwhile still permitting non-coherent processing to be used betweendomains. As shown in FIG. 9 , FFT bins may be established in both timeand frequency constraints with each set of adjacent bins representing afrequency “domain.” These FFT bins may have a frequency extentdetermined by the sampling rate of the signal, thus this techniqueassumes at least a portion of the transmitted signal is known (e.g. areference signal exists). Accordingly, the reference signal can beexpressed as “chunks” in both time and frequency.

With continued reference to FIG. 9 and further reference to FIG. 10 , inorder to set up these domains in the time constraint, first the highestDoppler frequency of concern should be determined based on maximumuncompensated velocity between the transmitter and the receiver. If theDoppler of the signal is known, then Doppler compensation may be usedprior to processing. According to one non-limiting example, if theDoppler has a 1600 m/sec maximum, then the Doppler at 950 MHz isapproximately equal to 5 KHz and at 1200 MHz is approximately equal to10 KHz. Next, a limit is placed on the amount of permissible driftwithin a domain. Typically, the drift within a domain should less than180°. For purposes of this example, assuming a drift limit of 90°provides that at the maximum Doppler (i.e. 10 KHz), the signal has aperiod of 100 microseconds. Then, the domains are further limited toapproximately one quarter, thus each time domain is approximately 25microseconds wide.

Next, the domains can be set using the frequency constraints as thefrequency bin size and FFT time extent are interrelated. First,reconsidering the maximum Doppler frequency offset, it is desirable tohave bins that are greater than that value by some degree. For example,typical values may be ¼ or 1/10 of the bin size; however, any value lessthan the drift limit may be used as dictated by the desired constraintsand implementation. Thus, using 1/10 and keeping with the example abovethen provides that each frequency bin should be at least 100 KHz widebefore taking the phase difference between bins into account. In orderto obtain a coherent correlation across bins, the relative Doppleroffset must be relatively small. According to one non-limiting example,if it is desired to have a less than 10° Doppler skew between the topand bottom bins in a domain, the useful values are likely between 1° and45°. Thus, if our coherence time bins are 25 microseconds wide, ourmaximum phase drift is 90° at 1200 MHz, and our minimum phase drift is45° at 950 MHz, the difference across the frequency band is 45°(90°−45°) which leads to 4.5 frequency domains to keep the skew to 10°or less, which can be rounded to 5 to further ensure the skew is belowthe desired maximum.

As mentioned above, frequency bin size and FFT time extent areinterrelated, so it is important to reconsider the Doppler offset to binsize frequency constraint in view of the maximum time domain constraint.Keeping with the presented example and assuming a 320 MHz sample rate,the samples in the FFT bins should be no more than 32000, which resultsin the previously discussed 100 KHz wide frequency bins; however 32000samples in the FFT results in a time extent of 100 microseconds, whichis too large, so the samples are limited to ¼ of that, or approximately8000 samples, to meet both the 25 microsecond time constraint and theDoppler to bin size offset constraint.

Now, with knowledge of the frequency and time constraints, the FFT sizemay be selected, which should be closest to the desired number ofsamples. In this example, and FFT size of 8192 would be the closest tothe 8000 samples that are desired and is efficient to implement. Giventhis, the FFT frame size is 25.6 microseconds which is close enough inproximity to the approximately 25 microsecond time constraint and ourfrequency bin size is 39.0625 KHz. Accordingly, with 250 MHz occupying6400 bins, each of our desired 5 coherence domains would each contain1280 bins, as best illustrated in FIG. 10 .

Now, with our coherence domains set up, the method of synchronizationusing these coherence domains will be provided by way of a non-limitingexample. For purposes of this example, assume an arbitrary chaoticsignal with transmitted sample values at certain times and frequenciesthat are well defined. According to one example, these known values maybe a TDMA preamble which may be used to synchronize the unknown contentin the signal. Once these unknown components are then estimated usingthe known values, the synchronization of the signal can be continuouslyupdated through employment of feedback loop 108 in a real-time, on-goingbasis. This example may further provide that synchronization parameterswithin coherence domains are estimated using coherent processing, whileparameters between domains are estimated using non-coherent processingtechniques.

The first step in the present coarse synchronization method is to detectand isolate the preamble in the received signal. This can provide afocus point for a more detailed channel estimation. Next, a series ofbulk correlations can be performed by multiplying two signals in thefrequency domain and then performing an inverse FFT, which can computecorrelations over multiple time shifts. The inverse FFT is bestperformed with coherent processing techniques.

From the preamble, time frequency reference tiles of size approximatelyequal to the size of the coherence domains can be created, with sometiles “padded” with zeros to permit bulk searches for the preamble witha larger FFT. Once the time frequency tiles are created, the FFT may beapplied thereto.

Additionally, the time frequency tiles may be further divided byfrequency to create the coherence domains, as discussed above. All binsthat do not fall into a given coherence domain can be zeroed and aseries of bulk correlations can be performed on the received signalusing coherent processing techniques within domains and non-coherenttechniques between domains, with magnitude added to the non-coherenttechniques.

There exists a presumption that overlapped FFTs may be less complex thanperforming a bulk correlation. Accordingly, in another example, thereference signal (i.e. the known preamble) can be broken intotime-frequency chunks with time blocks and frequency blocks. Prior thento transform, the time blocks can be padded with zeroes to increasetheir extent. Then, the FFT of the padded signal can be taken and thenfurther broken into frequency chunks.

Then, the received signal can be further analyzed for the preamble bycorrelating with each of the individual chunks by taking a chunk of thereceived signal, transforming it with an FFT, and then multiplying itwith the reference signals in the frequency domain. An inverse FFT maybe performed to find the time series correlation of each chunk of thereceived signal. These correlations can be time aligned and can havetheir powers summed before doing a peak detection. In this instance,some overlap between the signal chunks may provide some assurance thatthe preamble is not missed if it is divided between two chunks.

These steps may provide a coarse synchronization of the received signalwhich may be refined through re-using estimations from the signal viafeedback loop 108.

As described herein and mentioned above, the methods of waveformgeneration and signal processing address specific needs relating to LPxcommunications and are implemented using the described systems or othersuitable systems, as dictated by the desired implementation. Thesemethods increase the reliability, efficiency, speed, and cost parametersof LPx communications while maintaining low probability of detectionand/or exploitation of the signals described herein.

Various inventive concepts may be embodied as one or more methods, ofwhich an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. For example, embodiments of technology disclosed herein may beimplemented using hardware, software, or a combination thereof. Whenimplemented in software, the software code or instructions can beexecuted on any suitable processor or collection of processors, whetherprovided in a single computer or distributed among multiple computers.Furthermore, the instructions or software code can be stored in at leastone non-transitory computer readable storage medium.

Also, a computer or smartphone utilized to execute the software code orinstructions via its processors may have one or more input and outputdevices. These devices can be used, among other things, to present auser interface. Examples of output devices that can be used to provide auser interface include printers or display screens for visualpresentation of output and speakers or other sound generating devicesfor audible presentation of output. Examples of input devices that canbe used for a user interface include keyboards, and pointing devices,such as mice, touch pads, and digitizing tablets. As another example, acomputer may receive input information through speech recognition or inother audible format.

Such computers or smartphones may be interconnected by one or morenetworks in any suitable form, including a local area network or a widearea network, such as an enterprise network, and intelligent network(IN) or the Internet. Such networks may be based on any suitabletechnology and may operate according to any suitable protocol and mayinclude wireless networks, wired networks or fiber optic networks.

The various methods or processes outlined herein may be coded assoftware/instructions that is executable on one or more processors thatemploy any one of a variety of operating systems or platforms.Additionally, such software may be written using any of a number ofsuitable programming languages and/or programming or scripting tools,and also may be compiled as executable machine language code orintermediate code that is executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, USB flash drives,SD cards, circuit configurations in Field Programmable Gate Arrays orother semiconductor devices, or other non-transitory medium or tangiblecomputer storage medium) encoded with one or more programs that, whenexecuted on one or more computers or other processors, perform methodsthat implement the various embodiments of the disclosure discussedabove. The computer readable medium or media can be transportable, suchthat the program or programs stored thereon can be loaded onto one ormore different computers or other processors to implement variousaspects of the present disclosure as discussed above.

The terms “program” or “software” or “instructions” are used herein in ageneric sense to refer to any type of computer code or set ofcomputer-executable instructions that can be employed to program acomputer or other processor to implement various aspects of embodimentsas discussed above. Additionally, it should be appreciated thataccording to one aspect, one or more computer programs that whenexecuted perform methods of the present disclosure need not reside on asingle computer or processor, but may be distributed in a modularfashion amongst a number of different computers or processors toimplement various aspects of the present disclosure.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

“Logic”, as used herein, includes but is not limited to hardware,firmware, software and/or combinations of each to perform a function(s)or an action(s), and/or to cause a function or action from anotherlogic, method, and/or system. For example, based on a desiredapplication or needs, logic may include a software controlledmicroprocessor, discrete logic like a processor (e.g., microprocessor),an application specific integrated circuit (ASIC), a programmed logicdevice, a memory device containing instructions, an electric devicehaving a memory, or the like. Logic may include one or more gates,combinations of gates, or other circuit components. Logic may also befully embodied as software. Where multiple logics are described, it maybe possible to incorporate the multiple logics into one physical logic.Similarly, where a single logic is described, it may be possible todistribute that single logic between multiple physical logics.

Furthermore, the logic(s) presented herein for accomplishing variousmethods of this system may be directed towards improvements in existingcomputer-centric or internet-centric technology that may not haveprevious analog versions. The logic(s) may provide specificfunctionality directly related to structure that addresses and resolvessome problems identified herein. The logic(s) may also providesignificantly more advantages to solve these problems by providing anexemplary inventive concept as specific logic structure and concordantfunctionality of the method and system. Furthermore, the logic(s) mayalso provide specific computer implemented rules that improve onexisting technological processes. The logic(s) provided herein extendsbeyond merely gathering data, analyzing the information, and displayingthe results. Further, portions or all of the present disclosure may relyon underlying equations that are derived from the specific arrangementof the equipment or components as recited herein. Thus, portions of thepresent disclosure as it relates to the specific arrangement of thecomponents are not directed to abstract ideas. Furthermore, the presentdisclosure and the appended claims present teachings that involve morethan performance of well-understood, routine, and conventionalactivities previously known to the industry. In some of the method orprocess of the present disclosure, which may incorporate some aspects ofnatural phenomenon, the process or method steps are additional featuresthat are new and useful.

The articles “a” and “an,” as used herein in the specification and inthe claims, unless clearly indicated to the contrary, should beunderstood to mean “at least one.” The phrase “and/or,” as used hereinin the specification and in the claims (if at all), should be understoodto mean “either or both” of the elements so conjoined, i.e., elementsthat are conjunctively present in some cases and disjunctively presentin other cases. Multiple elements listed with “and/or” should beconstrued in the same fashion, i.e., “one or more” of the elements soconjoined. Other elements may optionally be present other than theelements specifically identified by the “and/or” clause, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, a reference to “A and/or B”, when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A only (optionally including elements other than B);in another embodiment, to B only (optionally including elements otherthan A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc. As used herein in the specification andin the claims, “or” should be understood to have the same meaning as“and/or” as defined above. For example, when separating items in a list,“or” or “and/or” shall be interpreted as being inclusive, i.e., theinclusion of at least one, but also including more than one, of a numberor list of elements, and, optionally, additional unlisted items. Onlyterms clearly indicated to the contrary, such as “only one of” or“exactly one of,” or, when used in the claims, “consisting of,” willrefer to the inclusion of exactly one element of a number or list ofelements. In general, the term “or” as used herein shall only beinterpreted as indicating exclusive alternatives (i.e. “one or the otherbut not both”) when preceded by terms of exclusivity, such as “either,”“one of,” “only one of,” or “exactly one of.” “Consisting essentiallyof,” when used in the claims, shall have its ordinary meaning as used inthe field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

When a feature or element is herein referred to as being “on” anotherfeature or element, it can be directly on the other feature or elementor intervening features and/or elements may also be present. Incontrast, when a feature or element is referred to as being “directlyon” another feature or element, there are no intervening features orelements present. It will also be understood that, when a feature orelement is referred to as being “connected”, “attached” or “coupled” toanother feature or element, it can be directly connected, attached orcoupled to the other feature or element or intervening features orelements may be present. In contrast, when a feature or element isreferred to as being “directly connected”, “directly attached” or“directly coupled” to another feature or element, there are nointervening features or elements present. Although described or shownwith respect to one embodiment, the features and elements so describedor shown can apply to other embodiments. It will also be appreciated bythose of skill in the art that references to a structure or feature thatis disposed “adjacent” another feature may have portions that overlap orunderlie the adjacent feature.

Spatially relative terms, such as “under”, “below”, “lower”, “over”,“upper”, “above”, “behind”, “in front of”, and the like, may be usedherein for ease of description to describe one element or feature'srelationship to another element(s) or feature(s) as illustrated in thefigures. It will be understood that the spatially relative terms areintended to encompass different orientations of the device in use oroperation in addition to the orientation depicted in the figures. Forexample, if a device in the figures is inverted, elements described as“under” or “beneath” other elements or features would then be oriented“over” the other elements or features. Thus, the exemplary term “under”can encompass both an orientation of over and under. The device may beotherwise oriented (rotated 90 degrees or at other orientations) and thespatially relative descriptors used herein interpreted accordingly.Similarly, the terms “upwardly”, “downwardly”, “vertical”, “horizontal”,“lateral”, “transverse”, “longitudinal”, and the like are used hereinfor the purpose of explanation only unless specifically indicatedotherwise.

Although the terms “first” and “second” may be used herein to describevarious features/elements, these features/elements should not be limitedby these terms, unless the context indicates otherwise. These terms maybe used to distinguish one feature/element from another feature/element.Thus, a first feature/element discussed herein could be termed a secondfeature/element, and similarly, a second feature/element discussedherein could be termed a first feature/element without departing fromthe teachings of the present invention.

An embodiment is an implementation or example of the present disclosure.Reference in the specification to “an embodiment,” “one embodiment,”“some embodiments,” “one particular embodiment,” “an exemplaryembodiment,” or “other embodiments,” or the like, means that aparticular feature, structure, or characteristic described in connectionwith the embodiments is included in at least some embodiments, but notnecessarily all embodiments, of the invention. The various appearances“an embodiment,” “one embodiment,” “some embodiments,” “one particularembodiment,” “an exemplary embodiment,” or “other embodiments,” or thelike, are not necessarily all referring to the same embodiments.

If this specification states a component, feature, structure, orcharacteristic “may”, “might”, or “could” be included, that particularcomponent, feature, structure, or characteristic is not required to beincluded. If the specification or claim refers to “a” or “an” element,that does not mean there is only one of the element. If thespecification or claims refer to “an additional” element, that does notpreclude there being more than one of the additional element.

As used herein in the specification and claims, including as used in theexamples and unless otherwise expressly specified, all numbers may beread as if prefaced by the word “about” or “approximately,” even if theterm does not expressly appear. The phrase “about” or “approximately”may be used when describing magnitude and/or position to indicate thatthe value and/or position described is within a reasonable expectedrange of values and/or positions. For example, a numeric value may havea value that is +/−0.1% of the stated value (or range of values), +/−1%of the stated value (or range of values), +/−2% of the stated value (orrange of values), +/−5% of the stated value (or range of values), +/−10%of the stated value (or range of values), etc. Any numerical rangerecited herein is intended to include all sub-ranges subsumed therein.

Additionally, the method of performing the present disclosure may occurin a sequence different than those described herein. Accordingly, nosequence of the method should be read as a limitation unless explicitlystated. It is recognizable that performing some of the steps of themethod in a different order could achieve a similar result.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures.

In the foregoing description, certain terms have been used for brevity,clearness, and understanding. No unnecessary limitations are to beimplied therefrom beyond the requirement of the prior art because suchterms are used for descriptive purposes and are intended to be broadlyconstrued.

Moreover, the description and illustration of various embodiments of thedisclosure are examples and the disclosure is not limited to the exactdetails shown or described.

The invention claimed is:
 1. A method of waveform synchronizationcomprising: receiving a signal via at least one antenna and at least onereceiver, the signal having a known preamble and wherein the data usedin generating the signal are not known except for the data symbols inthe preamble; creating a plurality of coherence domains using time andfrequency constraints to accept maximum uncompensated Doppler values;establishing a plurality of FFT bins in each of plurality of coherencedomains; performing a series of bulk correlations on the received signalby multiplying the received signals with a reference signal derived fromthe known preamble in the frequency domain and applying an inverse fastFourier transform (FFT) to the result to estimate the correlation valuesat points in time; detecting and isolating the preamble of the receivedsignal; estimating the synchronization values of the received signalwithin the coherence domains based on the known preamble; estimating thesynchronization values of the received signal between coherence domainsbased on the known preamble; and updating the synchronization values ofthe received signal on an ongoing and real time basis using theestimated synchronization values from the detected signal from a postprocessing feedback loop.
 2. The method of claim 1 wherein estimatingthe synchronization values of the received signal within coherencedomains is performed using coherent processing techniques.
 3. The methodof claim 2 wherein estimating the synchronization values of the receivedsignal between coherence domains is performed using non-coherentprocessing techniques.
 4. The method of claim 1 wherein creating theplurality of coherence domains further comprises: calculating a sizeconstraint in the time domain based the highest Doppler frequency ofconcern determined from a maximum uncompensated velocity between atransmitter generating the detected signal and the receiver.
 5. Themethod of claim 4 wherein the value of the size constraint in the timedomain is approximately one quarter of a period of the highest Dopplerfrequency of concern.
 6. The method of claim 4 wherein creating theplurality of coherence domains further comprises: calculating a sizeconstraint in the frequency domain based on a maximum Doppler frequencyoffset that is less than the FFT bin size determined by a drift limit ofless than 180°; and calculating the number of coherence domains requiredto keep a Doppler skew below 10°.
 7. The method of claim 6 wherein thesize constraint in the frequency domain is approximately one tenth ofthe FFT bin size determined by the drift limit.
 8. The method of claim 6wherein creating the plurality of coherence domains further comprises:calculating a number of signal samples to be taken per bin based on thesmaller of the size constraint in the time domain and the sizeconstraint in the frequency domain; and sampling the signal.
 9. Themethod of claim 8 wherein establishing the plurality of FFT bins furthercomprises: selecting an FFT size closest to the number of samples perbin; and determining the number of bins per coherence domain.
 10. Amethod of secure communications within an environment comprising:generating a communications signal with at least a portion of the signalbeing known and at least a portion of the signal being unknown;transmitting the communications signal from a first platform via atleast one antenna in operable communication with at least onetransmitter; receiving the communications signal with a second platformvia at least one antenna in operable communication with at least onereceiver; creating a plurality of coherence domains using time andfrequency constraints to accept maximum uncompensated Doppler values;establishing a plurality of FFT bins in each of plurality of coherencedomains; estimating the synchronization values of the communicationssignal within the coherence domains based on the known portion of thecommunications signal; estimating the synchronization values of thereceived signal between coherence domains based the known portion of thecommunications signal; and updating the synchronization values of thecommunications signal on an ongoing and real time basis using theestimated synchronization values from the detected signal from a postprocessing feedback loop.
 11. The method of claim 10 wherein estimatingthe synchronization values of the received signal within coherencedomains is performed using coherent processing techniques.
 12. Themethod of claim 11 wherein estimating the synchronization values of thereceived signal between coherence domains is performed usingnon-coherent processing techniques.
 13. The method of claim 10 whereincreating the plurality of coherence domains further comprises:calculating a size constraint in the time domain based the highestDoppler frequency of concern determined from a maximum uncompensatedvelocity between the first platform and the second platform.
 14. Themethod of claim 13 wherein the value of the size constraint in the timedomain is approximately one quarter of a period of the highest Dopplerfrequency of concern.
 15. The method of claim 13 wherein creating theplurality of coherence domains further comprises: calculating a sizeconstraint in the frequency domain based on a maximum Doppler frequencyoffset that is less than the FFT bin size determined by a drift limit ofless than 180°; and calculating the number of coherence domains requiredto keep a Doppler skew below 10°.
 16. The method of claim 15 wherein thesize constraint in the frequency domain is approximately one tenth ofthe FFT bin size determined by the drift limit.
 17. The method of claim15 wherein creating the plurality of coherence domains furthercomprises: calculating a number of signal samples to be taken per binbased on the smaller of the size constraint in the time domain and thesize constraint in the frequency domain; and sampling the signal. 18.The method of claim 17 wherein establishing the plurality of FFT binsfurther comprises: selecting an FFT size closest to the number ofsamples per bin; and determining the number of bins per coherencedomain.
 19. A system comprising: at least one antenna; at least onereceiver in operable communication with the at least one antenna, the atleast one antenna and at least one receiver operable to receive andtransmit electromagnetic signals; at least one processor capable ofexecuting logical functions in communication with the at least oneantenna and at least one receiver; and at least one non-transitorycomputer readable storage medium having instructions encoded thereonthat, when executed by the processor, implements operations to generatea waveform, the instructions including: receive a signal via the atleast one antenna and the at least one receiver, the signal having aknown preamble and wherein the data used in generating the signal arenot known except for the data symbols in the preamble; create aplurality of coherence domains using time and frequency constraints toaccept maximum uncompensated Doppler values; establish a plurality ofFFT bins in each of plurality of coherence domains; perform a series ofbulk correlations on the received signal by multiplying the receivedsignals with a reference signal derived from the known preamble in thefrequency domain and applying an inverse fast Fourier transform (FFT) tothe result to estimate the correlation values at points in time; detectand isolate the preamble of the received signal; estimate thesynchronization values of the received signal within the coherencedomains based on the known preamble; estimate the synchronization valuesof the received signal between coherence domains based on the knownpreamble; and update the synchronization values of the received signalon an ongoing and real time basis using the estimated synchronizationvalues from the detected signal from a post processing feedback loop.